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Sleep is the state of natural rest observed throughout the Animal Kingdom , in all Mammals and Birds , and in many Reptiles , Amphibians , and Fish .http://www.sleephomepages.org/sleepsyllabus/fr-b.html In humans, other mammals, and many other animals that have been studied — such as Fish , Birds , Ants , and Fruit-flies — regular sleep is necessary for survival. The capability for arousal from sleep is a protective mechanism and also necessary for health and survival. CHARACTERISTICS Sleep is generally characterized by a reduction in voluntary body movement, temporary blindness, decreased reaction to external Stimuli , loss of Consciousness , a 70% reduction in audio receptivity, an increased rate of Anabolism (the synthesis of Cell structures), and a decreased rate of Catabolism (the breakdown of cell structures). PHYSIOLOGY Stages The measurement of eye movement during sleep is used to divide sleep into two broad types: Rapid Eye Movement (REM) and Non-rapid Eye Movement (NREM) sleep. Each type has a distinct set of associated physiological, neurological and psychological features. Sleep proceeds in cycles of REM and NREM phases. In humans, this cycle is approximately 90–110 minutes. Each phase may have a distinct physiological function. Drugs such as Alcohol and Sleeping Pill s can suppress certain stages of sleep (see '' Sleep Deprivation ''). This can result in a sleep that exhibits loss of consciousness but does not fulfill its physiological functions. In REM, the Brain is active and the body inactive, and this is when most dreaming occurs. REM sleep is characterized by an Electroencephalography (EEG) that has low voltage and mixed frequency, similar in appearance to the awake EEG. During REM sleep there is loss of skeletal Muscle Tone , and an active Sympathetic Nervous System . In NREM sleep, the body is active and the brain inactive, and there is relatively little dreaming. Non-REM encompasses four stages; stages 1 and 2 are considered 'light sleep', and 3 and 4 'deep sleep'. They are differentiated solely using EEG, unlike REM sleep which is characterized by rapid eye movements and relative absence of muscle tone. There are often limb movements, and Parasomnia Sleep Walking occurs in non-REM sleep. A Cyclical Alternating Pattern may sometimes be observed during a stage. Rechtschaffen and Kales originally outlined the criteria for staging sleep in 1969. The American Academy of Sleep Medicine (AASM) updated the staging rules in 2007. Both REM sleep and NREM sleep stages 3 and 4 are homeostatically driven; that is, if a human is selectively deprived of one of these, it rebounds once the person is allowed to sleep. This suggests that both are essential in the sleep process and its many functions. NREM NREM accounts for 75–80% of total sleep time in normal human adults, and consists of four stages according to the 2007 AASM standards:
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